This invention relates to the art of electrophotography, and more particularly to a new and improved electrophotographic charging system and method.
One area of use of the present invention is in charging a photoconductive surface in an electrophotographic machine, although the principles of the present invention can be variously applied. The typical office copy machine employs the Carlson method of electrophotography, Xerography, to produce a dry, plain paper copy of an original black and white or color document. According to the Carlson method, a photoconductive surface, which can be in drum, sheet, or belt form, is used to produce and store latent images produced from the original. The latent image is produced by a process which charges, i.e. electrifies, the surface of the photoconductor in the dark or high resistance state to a uniform voltage level, typically between 600 and 1000 volts, and then selectively exposes the surface with light from and in registry with the original. The exposure of areas on the surface to light will convert these areas to a lower resistance state to cause discharging of those areas to various voltage levels, the levels being dependent on the intensity of the light from the original. Thus, a latent image is formed which is made up of areas of high voltage corresponding to black levels on the original, medium voltage level areas corresponding to gray levels on the original, and low voltage level areas corresponding to white levels on the original. Subsequent process transfer the latent image to plain paper, using a developer in the form of toner, resulting in a finished copy on plain paper.
To produce good contrast and resolution quality copies, it is important that the system and method for charging the photoconductive surface produce stable charging characteristics as a function of time, temperature, humidity and copy machine age and wear. Furthermore, it would be highly desirable to produce stable charging characteristics independent of spacing variations between the charging device and the photoconductor surface. In addition, it is important to provide the foregoing in a manner which does not produce an undesirable amount of ozone. Related to the foregoing considerations are changes in the magnitude of the electrophotographic surface charging current.
It is, therefore, a primary object of this invention to provide a new and improved electrophotographic charging system and method.
It is a more particular object of this invention to provide such an electrophotographic charging system and method for use in copy machines and other electrophotographic imaging machines which will provide for stable charging the electrophotographic surface to produce stable quality electrophotographic images as a function of time, temperature, humidity, and air pressure.
It is a further object of this invention to provide such a system and method which increases the efficiency of electrophotographic charging used in copy machine and other electrophotographic imaging machines to reduce the cost of machine manufacturing.
It is a further object of this invention to provide such an electrophotographic charging system and method which reduces the amount of ozone produced in electrophotographic machine imaging processes so as to provide less biological hazard and less destruction of machine parts by the corrosive effects of ozone.
It is a further object of this invention to provide such a system and method which reduces the cost of manufacturing of copy and other electrophotographic imaging machines by providing stable electrophotographic surface charging which is independent of the spacing variation between charger and surface, thereby allowing lower tolerances on machine components such as drums, rollers and guides.
It is a further object of this invention to provide such an electrophotographic charging system and method which increases useful life of copy and other electrophotographic machines by making the electrophotographic image or copy quality independent of the variation in mechanical components of the machine due to wear which would cause variation in charger to electrophotographic surface spacing.
It is a further object of this invention to provide such a system and method which reduces the operating costs of a copy or other electrophotographic machine by reducing the maintenance costs required to keep the charging device clean by making the photoconductor charging process independent of the cleanliness of the charging device.
It is a further object of this invention to provide such an electrophotographic charging system and method which provides an electrical signal having a magnitude proportional to the electrophotographic surface charging current.
It is a further object of this invention to provide such a system and method which enables the magnitude of the electrophotographic charging current to be precisely controlled by means of an electrical control signal to provide for quantitative charging of the electrophotographic surface.
The present invention provides a system and method for applying charge to a photoconductive surface wherein an electrode is in proximity to the surface and spaced between the surface and a shield characterized by applying a voltage to the electrode such that current flowing in the electrode is the sum of surface charging current and shield current, utilizing the shield current to obtain an electrical signal proportional to the surface charging current and utilizing that signal to control the application of voltage to the electrode. The shield current is utilized by connecting a current summing node in a circuit including the electrode and the shield such that the shield current and the sum of shield current and surface charging current flow in different directions relative to the node and by obtaining from the node the signal proportional to surface charging current. The signal proportional to surface charging current is compared to an input control signal and the result of the comparison is utilized to control the application of voltage to the electrode.
The foregoing is accomplished by providing a controlled source of charging current having an output connected to the electrode and an input coupled to the shield and being responsive to a control input so that direct control of the charging current is provided by the control input. The controlled source of charging current comprises a high voltage supply having an output, a return input and a control input which operates such that variations in a signal applied to the control input cause variations in the output of the supply, means for coupling the supply output to the electrode, a current summing node connected to the return input, means for connecting the shield to the summing node, and control means having an output connected to the control input of the supply, a first input adapted to receive a control signal, and a second input connected to the summing node so that as the charging current varies, as represented by variations in the voltage at the summing node, the control means applies a signal to the control input of the supply and therefore controls the charging current as determined by the magnitude of the control signal.